Please help me find the solution. Rb411 ether1 shows no link

Hallo people I am trying to diagnose a problem that I have with a rb411.

The problem is that sometimes, usually after a week then the Ethernet port 1 of the rb411 will show that there is no link. But the poe still gives the rb411 power. And then sometimes after about 5 hours then the rb411 will show that Ethernet 1 is connected again.

I tried a few things already. I replaced the switch (unmanaged) with another one, I have replaced the ether cable that runs from the poe to the rb411.

Last week it did the same thing with the new switch (about every hour). So I put back the old one and it worked fine for a week.

Now I want to replace the switch with a rb750g, but it is so confusing.

It does sounds like a problem with the rb411, but then again it also can be the switch. Usually when this happens then only the Ethernet light of the switch port that is connected to the rb411 is off, but when I remove the Ethernet cable and plug it into another port then it go back online again.

Can one of the experts on the forum please tell me what they think might be the problem?

Router or the Switch?

The last time I had a problem like that it was the router.
Doing some tests I actually found the problem went away
after I used straight ethernet no POE and just plugged in the
power into the router

other test I did was put the router end into a POE as well and
made a power cable go from the POE to the router
That also worked.

Hellson